Ever wanted to create a new type of advanced field for gravity forms ?  for example, I needed to create a dimension field with two select boxes, limiting the choices available for selecting a dimension in centimeters and millimiters. Well it took me quite a while to find out that a lot of documentation on the web is outdated : the "simple field addon" published on git hub simply does not work on latest gravity forms (version 2.0.6 as of today)... Instead of trying to debug that one I found a better explanation of extending fields on gravity forms GF_FIELD documentation page. It turns out that extending that main class in a simple file that you can include in your theme (or create a plugin from it), works fine. here is a sample code taken from the standard time field, quickly transformed to a centimeter / millimeter select box. NO GUARANTEE that this works, it is in development, it still contains some code from the original time field,  I will post updates as I progress, feel free to use it and make your own plugin out of this one

<?php
 
if ( ! class_exists( 'GFForms' ) ) {
	die();
}


class GF_Field_Doublenumberselect extends GF_Field {

	public $type = 'doublenumberselect';

	public function get_form_editor_field_title() {
		return esc_attr__( 'doublenumberselect', 'gravityforms' );
	}

	function get_form_editor_field_settings() {
		return array(
			'conditional_logic_field_setting',
			'prepopulate_field_setting',
			'error_message_setting',
			'label_setting',
			'sub_labels_setting',
			'label_placement_setting',
			'sub_label_placement_setting',
			'admin_label_setting',
			'time_format_setting',
			'rules_setting',
			'visibility_setting',
			'duplicate_setting',
			'default_input_values_setting',
			'input_placeholders_setting',
			'description_setting',
			'css_class_setting',
		);
	}

	public function validate( $value, $form ) {
	 
	}

	public function get_field_input( $form, $value = '', $entry = null ) {

		$is_entry_detail = $this->is_entry_detail();
		$is_form_editor  = $this->is_form_editor();

		$form_id  = absint( $form['id'] );
		$id       = intval( $this->id );
		$field_id = $is_entry_detail || $is_form_editor || $form_id == 0 ? "input_$id" : 'input_' . $form_id . "_$id";

		$form_sub_label_placement  = rgar( $form, 'subLabelPlacement' );
		$field_sub_label_placement = $this->subLabelPlacement;
		$is_sub_label_above        = $field_sub_label_placement == 'above' || ( empty( $field_sub_label_placement ) && $form_sub_label_placement == 'above' );
		$sub_label_class_attribute = $field_sub_label_placement == 'hidden_label' ? "class='hidden_sub_label screen-reader-text'" : '';

		$disabled_text = $is_form_editor ? "disabled='disabled'" : '';

		$hour = $minute = $am_selected = $pm_selected = '';

		if ( ! is_array( $value ) && ! empty( $value ) ) {
			preg_match( '/^(\d*):(\d*) ?(.*)$/', $value, $matches );
			$hour        = esc_attr( $matches[1] );
			$minute      = esc_attr( $matches[2] );
			$the_rest    = strtolower( rgar( $matches, 3 ) );
			$am_selected = strpos( $the_rest, 'am' ) > -1 ? "selected='selected'" : '';
			$pm_selected = strpos( $the_rest, 'pm' ) > -1  ? "selected='selected'" : '';
		} elseif ( is_array( $value ) ) {
			$value       = array_values( $value );
			$hour        = esc_attr( $value[0] );
			$minute      = esc_attr( $value[1] );
		 
		}

		$hour_input   = GFFormsModel::get_input( $this, $this->id . '.1' );
		$minute_input = GFFormsModel::get_input( $this, $this->id . '.2' );
 
		$hour_placeholder_attribute   = $this->get_input_placeholder_attribute( $hour_input );
		$minute_placeholder_attribute = $this->get_input_placeholder_attribute( $minute_input );

		$hour_tabindex   = $this->get_tabindex();
		$minute_tabindex = $this->get_tabindex();
	 

	 
 

		$hour_label = rgar( $hour_input, 'customLabel' ) != '' ? $hour_input['customLabel'] : esc_html__( 'CM', 'gravityforms' );
		$minute_label = rgar( $minute_input, 'customLabel' ) != '' ? $minute_input['customLabel'] : esc_html( _x( 'MM', 'Abbreviation: Minutes', 'gravityforms' ) );
         $option1 = self::getNumberSelect($hour  );
	     $option2 = self::getNumberSelect($minute );
	$label1=" <label for='{$field_id}_1' {$sub_label_class_attribute}>{$hour_label}</label>";
	$label2=" <label for='{$field_id}_2' {$sub_label_class_attribute}>{$minute_label}</label>";
		 $select1 ="<select type='{$input_type}' maxlength='2' name='input_{$id}[]' id='{$field_id}_1' value='{$hour}' {$hour_tabindex} {$hour_html5_attributes} {$disabled_text} {$hour_placeholder_attribute}> {$option1}</select>$label1</i>";
         $select2 ="<select type='{$input_type}' maxlength='2' name='input_{$id}[]' id='{$field_id}_2' value='{$hour}' {$hour_tabindex} {$hour_html5_attributes} {$disabled_text} {$hour_placeholder_attribute}> {$option2}</select>$label2</i>";

		 
			return "<div class='clear-multi'>
                        <div class='gfield_time_hour ginput_container ginput_container_time' id='{$field_id}'>
                            
							{$select1}
                        </div>
                        <div class='gfield_time_minute ginput_container ginput_container_time'>
                            
							{$select2}                        
						</div>
                        {$ampm_field}
                    </div>";
	 
	}

	public function get_field_label_class(){
		return 'gfield_label gfield_label_before_complex';
	}
public function getNumberSelect($value)
{ 
 
	for ($i=0; $i<100; $i++)
		{
				$selected  =  ($value==$i?"selected":"");
				$str .="<option value='$i' $selected>$i</option>";
		}
	 
	return $str;
	 }
	public function is_value_submission_empty( $form_id ) {
		$value = rgpost( 'input_' . $this->id );
		if ( is_array( $value ) ) {
			// Date field and date drop-downs
			foreach ( $value as $input ) {
				if ( strlen( trim( $input ) ) <= 0 ) {
					return true;
				}
			}

			return false;
		} else {

			// Date picker
			return strlen( trim( $value ) ) <= 0;
		}
	}

	public function get_value_save_entry( $value, $form, $input_name, $lead_id, $lead ) {

		// if $value is a default value and also an array, it will be an associative array; to be safe, let's convert all array $value to numeric
		if( is_array( $value ) ) {
			$value = array_values( $value );
		}

		if ( ! is_array( $value ) && ! empty( $value ) ) {
			preg_match( '/^(\d*):(\d*) ?(.*)$/', $value, $matches );
			$value    = array();
			$value[0] = $matches[1];
			$value[1] = $matches[2];
			$value[2] = rgar( $matches, 3 );
		}

		$hour   = empty( $value[0] ) ? '0' : wp_strip_all_tags( $value[0] );
		$minute = empty( $value[1] ) ? '0' : wp_strip_all_tags( $value[1] );
		$ampm   = wp_strip_all_tags( rgar( $value, 2 ) );
		if ( ! empty( $ampm ) ) {
			$ampm = " $ampm";
		}

		if ( ! ( empty( $hour ) && empty( $minute ) ) ) {
			$value = sprintf( '%02d:%02d%s', $hour, $minute, $ampm );
		} else {
			$value = '';
		}

		return $value;
	}

	public function get_entry_inputs() {
		return null;
	}

 
	public function sanitize_settings() {
		parent::sanitize_settings();
		if ( ! $this->timeFormat || ! in_array( $this->timeFormat, array( 12, 24 ) ) ) {
			$this->timeFormat = '12';
		}
	}

}

GF_Fields::register( new GF_Field_Doublenumberselect() );